Step 1: Create a new pagepage by going Pages >> Add New
Step 2: Name the page and set Template in Page Attributes to Coming Soon 01 or Maintenance
Step 3: Publish page
Step 4: Navigate to Appearance >> Customize >> Maintenance >> General
Change Page to the page you have created already.
Step 5: Do not forget to hit Publish
Notice: you must log out to see Maintenance Page
Step 1: Go to Appearance >> Customize >> Maintenance
Step 2: Double kick on Coming Soon 01 or Maintenance
Step 3: You can customize Logo, Title, Text, Countdown and Mailchimp Form here.